The problem of electrostatic discharge (ESD) is well known. From merely receiving a mild shock after crossing a room and touching a metal object, to sending a shock into electronic equipment, nearly everyone has experienced an ESD problem at some time.
While static electricity is extremely complex, several overall theories are generally accepted with regard to the action of ESD. Static electricity charges on a person or object are generally like charges. As such, as static electricity charges build up on a person or object, these charges tend to migrate as far apart from each other as possible as determined by the geometry of the person of object. Thus, for example, it is common for static electricity charges to migrate to a person's fingertips. For this reason, when that person reaches out to touch an electrically conductive object, a spark will jump when the gap between that person's fingertips and the object based upon the potential difference between the fingertips and the object. This discharge is very rapid and can be quite violent. If the electrically conductive object is sensitive electronic equipment, that equipment may be damaged either from the magnitude of the discharge and/or from the speed of the discharge. At the least, the charge could cause the equipment to execute an error. A sufficient number of such discharges may eventually damage the equipment.
Accordingly, the art contains many inventions intended to protect the equipment or the person from the effects of this sudden, and sometimes violent, discharge associated with ESD.
For instance, in the logging industry where chains are lowered by helicopter to loggers waiting on the ground to fasten fresh-cut timber to them so it could be airlifted to the sawmill or nearby waterway, track access point or the like, the loggers are often reluctant to grab the chain because of a painful shock that may occur as a result of a buildup of static electricity which will be discharged to ground through their bodies. This particular problem has been solved by incorporating a resistance in the line from the charge-carrying object, such as the helicopter, to the person on the ground. The high resistance causes the current to be low enough that the discharge will not be painful.
However, this is cumbersome. This solution may be even more cumbersome if the person is an office worker who moves around a great deal. Accordingly, this solution to the ESD problem has serious shortcomings.
Accordingly, there is a need for a system that protects a person against the effects of ESD but can do so in a manner that does not interfere with any task the person may be performing and further will not be cumbersome or burdensome for the person to use.
Still other inventions are directed to protecting electronic equipment from the effects of ESD. For example, many computers include touch pads or touch areas for the user to touch before touching the remainder of the computer. The touch pads are grounded so the ESD will pass from the person via a spark or the like directly to ground without going to or through the computer.
While many of these devices work well, there are several problem areas not addressed thereby. This results in drawbacks and disadvantages for such devices when a person or equipment are situated in certain environments or subject to certain conditions.
First, no matter how effective a touch pad is it will be totally ineffective if the person does not use it. That is, if the person carrying a large ESD charge forgets to touch the touch pad and proceeds to touch a computer, the ESD will discharge through the computer and the touch pad will have been useless. Thus, a shortcoming of such touch pads is that they require the person to remember to use it.
Furthermore, no matter how effective the ESD protection device is, the current level and/or the change in current level may be so high that either the person or the equipment can be damaged.
Still further, while placing a touch pad on a computer may protect the computer it does not protect the user from the effects of an electrostatic discharge.
As mentioned above, the majority of applications for the prevention of ESD are in the manufacturing or medical fields and are largely concerned with protecting the ‘manufacturing’ process or sensitive components for ESD damage. Examples include moving mediums such as the manufacture of rolls of paper, the assembly of delicate electronic chips and circuitry and surgeon-patient contact during an operation.
An analysis of each of the above will help illustrate the shortcomings of the prior art. In the manufacture or printing of paper, long rolls of paper may move at high speed. Often the path may involve rubber or other rollers and guides. As the paper rubs across such items a static electricity charge may be generated. Since the paper path is well controlled, it is an easy process to place grounded conductive brushes or flat metal springs in contact with the moving paper since the paper stays in a fixed path. Such electrodes are connected directly to the grounded frame of the associated machinery or to another path eventually leading to earth ground or other equalizing means.
Another common application of ESD control is in the production or repair of fragile electronics such as computer circuit boards. Even a slight electrostatic discharge through a sensitive device may destroy it. Therefore, significant effort and cost is devoted to eliminating the possibilities of electrostatic potentials in the vicinity of the sensitive electronics. Typically, a single ground point is provided that all associated elements are connected to so that no electrostatic potential can exist between them that might flow through the sensitive electronics. For example, a conductive floor mat is provided that is connected to the ground point, or a work surface mat that is conductive (or dissipative) is also provided that is wired to the same ground point, the work table frame and any test equipment is connected to the same point, finally the assembly person is also connected to the same point, typically by a wrist strap tether. The tether generally consists of a wrist pad and grounding wire that is eventually connected back to the single ground point. For operator safety, the ground wire typically contains a 1 Meg resistor to limit current flow to safe levels should the operator come in contact with 120 volts AC. This tethering restraint is inconvenient and not considered suitable for a typical office worker or call center operator. The single ground point is eventually connected to true earth ground or other equalizing point by another conductor.
Applications are similar in the medical field, employing similar tethers and/or foot/shoe connectors also considered impractical for the typical office worker environment.
Today, a new set of ESD problems is emerging in the typical work place or home office environment. Today, a typical worker may exist in a virtually electrically isolated environment—a plastic computer case, plastic keyboard, plastic control knobs on a molded plastic control panel, plastic office chair with man-made fabric and plastic wheels, non-conductive flooring or carpeting and even a headset with foam or molded plastic earpieces and plastic microphone tube.
As the operator moves in his/her chair, there are many opportunities for a very large electrostatic charge to build up on his/her body. Friction between dissimilar materials is the classical means for generation of electrostatic voltages. There are many such situations that exist continually in the operator environment today-the operator's clothing sliding against the chair back or arm rests, the operator's shoes sliding on the carpet, the plastic chair wheels sliding against the carpet are a few examples. The effects can be cumulative over a long period of time, and can become quite high.
Eventually a discharge or equalization to (true earth) ground must take place. The higher the value of the electrostatic voltage charge, the greater the distance the charge may ‘jump’ to discharge, and the more ‘catastrophic’ the event to the operator. For example, there are many documented cases of operators in call centers experiencing a very loud pop or explosion in their ear, ear pain, and even bleeding in the ear as the discharge path appears to take place through the operator's headset. Other documented cases include severe neck pain, nausea, numbness, elevated blood pressure and rapid heart beat.
There are many possibilities as to why these effects are worse than the typical nuisance static electricity charges walking around the house. For instance, the discharge path may be more surprising or appear worse to the user if it involves the user's ear. Recently, this has been attributed to electrostatic discharge of the operator with the grounding mechanism being the metallic portion of the ear piece coupled to its metallic conductors and eventually to earth ground through its associated electronics. This may be a direct low impedance ground or it might be a higher impedance which is still sufficiently low with respect to that needed to successfully equalize the static charge. Still in other cases, as explained below, the associated electronics may potentially make the discharge injury to the person more severe and disturbing by causing a high current pulse to take place as the discharge event. This effect may be further compounded by allowing the operator to be exposed to other voltage or leakage paths developed via the ear over time.
In some cases, the associated electronics may experience physical damage or processing disturbances due to the operator electrostatic discharge. For example, the headphone circuit might involve a transformer with a 600 to 10,000 volt breakdown rating between its windings (connected to the headset diaphragm) and conductive metal core. However, the electrostatic voltage on the operator may exceed 15,000 volts-far more than the design tolerance of the transformer. Should the transformer be exposed to such excessive high voltage, a ‘breakdown’ or ‘shorting’ may occur. Thus, the operator electrostatic voltage might cause a ‘short circuit’ insulation breakdown or lower resistance to develop between the headset winding (secondary) and primary winding which may be at a constant high voltage level with respect to ground or the transformer core which may be connected to earth ground, thus completing the discharge path.
The transformer breakdown may cause a permanent equipment failure. Other equipment damage or errors can also occur due to the electrostatic discharge event. The electrostatic discharge event may cause an electromagnetic or radio frequency pulse to be generated. This pulse may radiate into nearby circuitry causing errors in processing or noise in audio or video circuits. Although a transformer discharge event has been described above, other similar discharge paths can be envisioned, with similar catastrophic results.
With continued miniaturization of electronics, the problems may become more severe as circuit component voltage tolerances become less and enclosure insulation distances become less.
Accordingly, there is a need for an ESD protection system that protects a person and electronics from the effects of ESD, even if that person is in an environment that is intended to nominally insulate that person from ground.
As the cost of doing business increases, many businesses are reluctant to purchase new original equipment. Thus, it is most advantageous if existing equipment can be easily modified or retrofit to achieve new and improved results. This is the situation with protecting people from the effects of ESD. Thus, there is a need for a system for protecting people against the effects of ESD that can easily be retrofit onto existing equipment.
The parent application Ser. No. 09/934,047, the disclosure of which is incorporated herein by reference, discloses an overall system for overcoming the above-discussed problems.
In addition to the above-discussed problems, there are many situations that require a means for conveniently grounding electronic equipment beyond the safety requirements of the National Electric Code (NEC). Not only is grounding required to overcome problems associated with ESD, grounding is often required for safety reasons. The NEC requires that all new general purpose receptacles, for example, contain a grounded conductor.
Therefore, there is a need for a means to properly ground elements to not only overcome problems associated with ESD, but to meet NEC requirements as well.
Typically, the ground used to satisfy NEC requirements is the “round” hole on the standard home or office receptacle and many plugs have a prong to make contact with that ground element to provide safety grounding to attached equipment. Like the safety provisions of a high voltage conductor and prong, this grounding prong also is protected from casual access when a plug is plugged into the receptacle. Thus, there is no convenient access to the ground conductor for many common situations.
Therefore, there is a need for a means for providing convenient access to the ground conductor of many receptacles.
The need for grounding can be illustrated by two examples: the home theater and the above-discussed ESD fields. In the home theater, most devices such as a source (FM tuner, DVD player, and the like) are connected to preamplifiers or even amplifiers by inexpensive shielded cables. In some cases, such as when amplifiers are co-located with speakers, these cables can become quite long and the signals are very low level. Thus, they are susceptible to “hum” and “noise” being picked up over the cable. Often, the source of the hum or noise will be from currents traveling along the ground conductors of the cable (typically the shield) and coupling the noise into the signal path. The currents may originate because of lack of consistent grounding on all connected devices. This may be because different outlets are used, extension cords without a third wire ground are used, or many audio/video components come with only a two-prong plug without a ground which is typically a third prong. There are almost always minute currents in such devices wanting to travel to true ground. Thus, if one device is grounded and others are not, ground “leakage” currents will tend to flow through the signal cables to the grounded device and in the process cause what is typically called sixty cycle hum or other noise. This leakage ground current situation is illustrated in FIG. 1. There may be ungrounded source devices UD such as DVD players or VCRs. A central preamplifier PA, common to many ungrounded devices may be grounded as shown by the third wire TW on a power plug PP. Leakage currents, li, from the ungrounded devices flow along a common (shield) conductor CC of the signal cable and can be cross-coupled into the signal path. Charge built up by ESD can also flow in the same manner.
In many cases, the lack of a convenient fool-proof ground leads to a potentially dangerous or lethal situation. One typical situation is in the ESD field, although it will be understood by those skilled in the art that other situations can occur, includes a person purchasing a ring terminal and using it incorrectly. The inventor has been aware of ESD equipment distributors improperly advising their customers with the result being dangerous and frightening electrical arcing taking place along with potentially lethal situations.
As mentioned above, one common ESD application is to provide personnel with a grounded wrist strap to ground their body, thus dissipating any electrostatic charge built up in their body before it might damage sensitive electronic equipment they may be repairing or building. A grounding kit K is shown in FIG. 2 and might include a ground wire GW leading to a disconnect plate DP. The ground connection provided for the wire is typically a bare exposed metal ring terminal RT. Disconnect plate DP, shown in end view in FIG. 2, may be mounted under a front edge of a user's work table so that a wrist strap WS connection can be easily connected or disconnected. Typical “banana plugs” BP are used for the connection at the disconnect plate. These are designed for signal, not electrical power, applications and hence have none of the safety features required for electrical power connectors. Connections are typically exposed on both the front and the rear of the plate. If these connections become charged with the typical one hundred twenty volts of commercial electrical power, it could be dangerous to the user. In addition, the wrist strap to the user generally contains a one meg resistor in its electrical path to limit current to a safe value should the source wire come in contact with one hundred twenty volts. Somehow, if someone wired around the resistor or the strap or grounding element does not contain a resistor a potentially lethal situation could exist if the source comes in contact with one hundred twenty volts. The ground wire on these kits typically is terminated with an uninsulated ring terminal. Ground wires with similar terminals are sold in many other consumer situations such as to ground computer monitor shields.
It is an improper application of this ring terminal which leads to many dangerous situations. It is designed to be installed under a grounded screw in a safe manner by a qualified electrician or other such person skilled in the art. If, for example, a ring terminal is merely placed on the ground prong of a three prong plug, contact could be made with the hot conductor but not ground. FIG. 3A shows an end view of a typical plug P and ring terminal RT′. In the case illustrated in FIG. 3A, high voltage from one of the connectors C is exposed in a dangerous manner at the disconnect plate via ring terminal RT′ and subsequent locations such as the user's wrist strap or other devices which are believed by a user to be grounded. It is also conceivable that a user might simply attempt to stick the ring terminal into the “hot” conductor hole of the receptacle or two wire extension cord resulting in a dangerous situation.
Correct use of the ring terminal would be to place it under a ground screw. Often, this is the center screw of the cover plate of a standard duplex receptacle or screw on the metal body of a computer case or grounded piece of electrical equipment. However, today there are fewer availabilities of such ground screws. Most of the receptacles used in the millions of work stations today are a combination of press fits and molded assemblies without exposed ground screws. An example of such a situation is illustrated in FIG. 3B for receptacles MR. Thus, there is no easy way to properly ground in the field in many environments.
Thus, there is a need for a device for easily properly grounding in the field in most work environments.
Most computer cases today are plastic without exposed ground screws on the case. However, most connectors on the computer, such as the mouse plug and receptacle contain a ground conductor but there is no convenient way to use it for general or special-application grounding purposes.
Some applications, such as electrostatic discharge, require grounding to eventually discharge a static charge to true ground. The average current flows are very low and the impedance to ground can be quite high, well in the megohm range. This affords many additional opportunities for ground paths that would not be acceptable for or meet the requirements of typical electrical safety grounding where the ground system must be capable of supporting tens or hundreds of amps until a circuit breaker can shut down. For purposes of this disclosure, such paths will still be referred to as ground paths.